Single-phase AC motors usually comprise a rotor and stator composed of primary and secondary windings. The secondary winding is not only used to start the motor but also work during normal operation of the motor. Therefore, the complete secondary winding circuit of a single-phase AC motor can be represented with a secondary winding operation circuit and a secondary winding starting circuit shunted to each other. A motor in which the secondary winding doesn't work during normal operation of the motor doesn't have the secondary winding operation circuit portion. Thus, in this type of motor, the secondary winding starting circuit portion only works at startup of the motor and has to be disconnected after motor startup.
In conventional motors, disconnection of the secondary winding starting circuit is usually implemented with a Positive Temperature Coefficient (PTC) component. When the motor starts, the starting circuit works, and the starting current in the secondary winding flows through the PTC component, causing the PTC component to generate heat and thus the resistance increases quickly, and finally disconnects the starting circuit. During normal operation of the motor, there is a small current passing through the PTC component persistently, to maintain the thermal resistance of the PTC component and thereby prevent the motor starting circuit from acting. The power loss for maintaining heat generation in the PTC component is usually about 3 W. Since such motors are widely used, the heat loss results in severe waste of electric energy.
In China Patent Application CN1052228A, an electronic circuit for starting single-phase inductive motors is disclosed. By utilizing a triac that gates on in normal state and gates off by timing in the motor starting circuit and associated gate-on/gate-off circuits, the switching of motor starting circuit from close at power-on to time-lagged open is implemented, and thereby motor startup is implemented. However, the amount of discrete components employed by the circuit has an extremely high demand for reliability in the typical application case, in which the motor has to start intermittently and frequently during long term operation, and it reduces the reliability of starting circuit to a great extent; especially, the working current of several milliamperes passes through the resistance components used in the gate-on circuit and multiple transistors used in the time-lagged gate-off circuit. Furthermore, the entire electronic circuit operates in the complex permanently operating circuit and is not isolated from the complex electrical environment effectively. As a result, the entire weak light-current circuit system is not protected well, bringing severe adverse effects to reliability of the entire motor system. In addition, the triac used in the circuit requires resetting the timer before the motor starting circuit is closed, i.e., the timer circuit must be cut off power so that the capacitor of the timer discharges for a certain time before the triac can gate off again after a time lag. Otherwise the triac will be always in gate-off state as long as there is current in the gate-on/time-lagged gate-off circuits, thereby preventing motor switching from non-operation low-current state to start state.
The invention disclosed in Japan Patent Document JP10094279 utilizes the principle that the current in a motor during startup is different to that in normal operation and utilizes a current detection resistor in the main motor circuit to convert the main circuit current (total current in motor) signal into a voltage signal. In addition, that invention utilizes a “reference current setter” and a “current comparison circuit” to compare the main circuit current signal with the preset current value, and uses the comparative result to control gate-on/gate-off of the triac connected in the motor starting circuit, and thereby attain the object of control open/close of the starting circuit. However, the resistor used in the circuit samples the current signal in the main motor circuit into a voltage signal. Since the resistor is a pure resistive component and the current in the main motor circuit is usually at ampere level or near ampere level, the resistor will cause heat loss at watt level or near watt level during the entire motor operation process. Though the circuit can start the motor, it degrades energy utilization ratio severely. Furthermore, the circuit utilizes auxiliary circuits such as “reference current setter” and “comparison circuit”, all of which degrades reliability of motor startup function to a certain degree.